Gas Exchange and Water Relations of the Root Hemi-parasite Santalum album L. in Association with Legume and Non-legume Hosts

This paper examines foliar nitrogen (N) levels, photosynthesis, transpiration, water use efficiency and tissue water relations of the xylem-tapping root hemi-parasite Santalum album in pot culture with various N-2-fixing woody hosts, a non-fixing host (a eucalypt), or in the absence of a host. Foliar N concentrations of Santalum were significantly greater than corresponding hosts and higher when on N-2-fixing hosts than on the eucalypt, or without a host. Strong positive relationships were evident in Santalum between foliar N concentration, rates of net photosynthesis and instantaneous water use efficiencies. Photosynthesis rate and water use efficiency of Santalum were generally lower than in corresponding hosts, but transpiration rates were not noticeably different between associations. delta(13)C values of total shoot dry matter of Santalum were poorly correlated with instantaneous water use efficiency as measured by gas exchange, but associations involving the three legumes showed less negative delta(13)C values and better water use efficiencies for hosts, than corresponding parasites. Interpretation of such differences was difficult in view of an earlier demonstration of substantial heterotrophic gain of C from certain hosts. Diurnal profiles of gas exchange and leaf water potential of hosts and parasites indicated closely coordinated diurnal stomatal responses of the parasite water relations to its host, thus resulting in transpiration rates of the parasite generating leaf water potential gradients favouring continuous abstraction of water and nutrients from a host. Tissue water relations of Santalum generally resembled those of water-stress tolerant species. Host-specific effects on relative water content and osmotic adjustment were slight and rated unimportant in regulation of water flow to the parasite, or in protecting it from temporary water stress in a host. (C) 1999 Annals of Botany Company.